It is known art realizing such a type of synchronizing device, defined in the art as “personal-weighing”, by hinging the seat frame to a support integral with the chair base by two cranks, usually reciprocally parallel, conveniently shaped and hinged to the afore said support, such that their rotation cause a corresponding rotation of the seat frame relatively to the ground. The crank rotation may be caused indirectly by the frame rotation of the chair back, constrained to one of the cranks, or to the seat frame, by an articulated system.
For example, the International Patent Application WO-A-2008/149224 in the name of MALENOTTI, describes such a device, wherein the frame of the chair back is constrained to the seat frame by a leverage causing the roto-translation on such a seat frame, when the user causes the rotation of the frame of the chair back itself. The shape of the four-bar linkage constraining the seat frame to the support integral with the base is of the type allowing such a roto-translation, leading the seat from a starting position, substantially horizontal, with the chair back erected, to a final position, with the chair back reclined at the rear, wherein the seat is withdrawn towards the chair back and its front portion is lifted, even if to few degrees, relatively to its rear portion.
Such a MALENOTTI device, while allowing an efficient synchronization of the chair back tilt with the seat tilt, is mechanically complex and then markedly burdensome to manufacture.
In addition, the device described in the Application WO-A-2008/149224 does not allow obtaining, during the device designing and producing steps, desired changes in the initial or final tilt angle of the seat simply, because of the multitude of components that would be newly designed and replaced.
Alternatively to such a device, it is known to realize a seat wherein the seat tilt, while the chair back tilt is varying, is controlled by a lever with two arms, wherein one arm is integral with the frame of the chair back, one arm is acting on the rear end of the seat frame, and the whole lever has a fulcrum on the afore said support integral with the seat base.
Such a solution, wherein the front portion of the seat frame is constrained to the base support by a rod hinged to the same support and integral with such a front portion of the seat frame, is described in the French Application FR-A-2045120 in the name of DUPART.
The DUPART device, although extremely simple, provides that while the tilt angle of the chair back is increasing, relatively to its rest position, the rear portion of the seat would raise relatively to the corresponding front portion, with a certain worsening of the posture achievable by the user, when the latter would like to lean against the chair back.
It is therefore an object of the present invention to realize a device for synchronizing the tilt of a back and seat of a chair of the type wherein while the tilt angle of the chair back is increasing, the corresponding raising of the front portion of the seat is occurring relatively to the rear portion, that would not produce the known art drawbacks and thereby being structurally simple and easy and efficient to set up. Another object of the present invention is to provide a device, easily adjustable in the designing step, that would not be expensive to manufacture and at the same time allowing the user to find a correct posture relating to his/her own weight.